Extrusion of thermoplastic tubes



June 14, 1966 J. F. E. ADAMS 3,

EXTRUSION OF THERMOPLASTIC TUBES 2 Sheets-Sheet 1 Original Filed April10, 1962 Inventor JOHN F. a. ADAMS y 0nd torneyfi June 14, 1966 J. F. E.ADAMS 3,255,377

EXTRUSION OF THERMOPLASTIC TUBES Original Filed April 10, 1962 3Sheets-Sheet 2 I nventor JOHN E E. ADAMS B wmmm Attorneys United StatesPatent 61 3 Claims. (Cl. 264-209) This application is a division ofapplication Serial No. 186,399, filed April 10, 1962, now abandoned.

This invention relates to the extrusion of synthetic thermoplastic tubeparticularly in the production of thermoplastic bottles formed by theexpansion of a softened length of extruded tube which is enclosed in amould and expanded to the internal configuration of the mould by theintroduction of fluid pressure into the enclosed tube length.

It is an object of the present invention so to effect the extrusion of asoftened length of tube for enclosure in a mould and expansion thereinas to control the weight per unit length of the material forming thetube, and it is a further object so to-form the tube for a bottle thatthe tube portion which is to form the neck of the bottle has a diametersuch that it will be housed in the neckforming portion of the mouldwithout being pinched by the mould, as has hitherto been necessary,while the main body of the bottle can be formed from a tube portionwhich is of relatively large diameter.

According to the invention there is provided the method of producingfrom synthetic thermoplastic material a parison for inflation in amould, said parison consisting of a tube the weight per unit length ofwhich varies at predetermined positions lengthwise of the tube, byextruding the material through an extrusion orifice at a constant rateand periodically reducing the weight per unit length of the tube bydecreasing longitudinal contraction of the material in the direction ofextrusion by subjecting the material to a circumferential shearingaction in one direction to create residual hoop stresses in the materialabout to issue through the orifice.

In order that the invention may be clearly understood one embodimentthereof will now be described, by way of example, with reference to theaccompanying diagrammatic drawings, in which:

FIG. 1 is a diagrammatic sectional elevation of apparatus according tothe invention,

FIG. 2 illustrates the formation of a length of tube extruded by theapparatus for enclosure in a mould, and

FIG. 3 diagrammatically illustrates an alternative form of theapparatus.

Referring to the drawings, an extrusion machine 1 of any suitable knownkind for extruding molten synthetic thermoplastic material, such aspolyethylene, has fitted thereto a die which includes a housing 2 havinga passage 3 through which molten synthetic thermoplastic material is fedto the die.

An annular cover 4 is secured to the outlet end of the housing 2 andtogether with a core 5 which is co-axial therewith forms an annularextrusion orifice 6 through which the molten material is extruded as atube. I

The core 5 is mounted for rotation about the axis thereof in bearings 7carried by the die housing 2 and to the end of the core remote from theend thereof which co-operates with the cover 4 to form the annularextrusion orifice 6 there is secured a pinion 8. The pinion 8 mesheswith a toothed rack or segment 9 which is connected with operating meanswhereby rotation of the core 3,256,377 Patented June 14, 1966 "ice 5 canbe selectively effected for predetermined intervals. With this form ofapparatus the core 5 can be rotated in one direction during the creationof a shearing action in one portion of molten material passing throughthe die, and in the reverse direction while another portion of thematerial is passing through the die. In one embodiment of the inventionthe operating means comprises a pneumatic device formed by an aircylinder 10 housing a piston 11 the connecting rod 12 for which isconnected to the rack or segment 9 to efiect operation thereof.

In operation, the die functions in the customary known manner during theextrusion of that portion 13, FIG. 2, of a tube length which is to formthe main body of a bottle or other article which is formed by expansionof the tube length while it is enclosed in a mould, and the tube tendsto swell as it leaves the extrusion orifice as indicated at 14, FIG. 2.During the extrusion of that part 15 of the tube length which is to formthe neck of a bottle the core is rotated relative to the cover and by sodoing induces in one direction a shearing action around thecircumference of the material passing through the extrusion orifice 6.The shearing action forms in the material a residual hoop stress whichas the material leaves the die causes contraction of the tube towardsthe axis thereof thereby forming a tube portion 15 which is of lesserexternal diameter than that of the normally extruded portion 13 of thetube length and which has a greater wall thickness than that of thenormally extruded portion of the tube length. The timing of the rotationof the core 5 is such that it occurs when and while the molten materialpassing through the die is that portion of the material which is to formthe neck of the bottle, and the contraction effected as the materialleaves the die, and due to said residual hoop stress, is such that theneckforming portion 15 of the tube can be housed in the neckformingportion 16 of the mould without being pinched thereby. In one embodimentof the invention in which the core is rotated during the extrusion ofmaterial which is to form the neck of a bottle, the core is rotated forapproximately one quarter of a turn at a rate of about 20 rpm.

The mould is a sectional mould comprising sections 17, 18 supported inknown manner, not shown, for movement towards and away from each other,and, if desired for movement towards and away from the die along theaxis of extrusion. Means, also not shown, known per se are provided toeffect expansion of a tube length to conform it to the shape of theinterior of the closed mould. Cutting means, not shown, of known kindare arranged to sever the tube length enclosed in the mould from theparent tube being extruded through the orifice 6.

It will be understood that, if desired, the relative rotational movementbetween the core 5 and cover 4 can be effected by rotation of the coverinstead of the core as described above, or alternatively both the coreand the cover may be rotated in the same direction in which instance thecore will be rotated at a different angular velocity from that at whichthe cover is rotated in order to induce the said circumferentialshearing action in one direction in the material passing between thecore'and cover.

Further, if desired, the diameter of the extruded tube may be controlledby rotating the core continuously in one direction by a driving gear 19,FIG. 3, the reduction in diameter of the tube and the thickness of thetube wall being proportional to the rate of rotation of the core, androtation of the core .can be controlled, for example, by a variablespeed driving mechanism 20, to obtain cyclic variations of the rate ofrotation and periods of rest.

Distribution of the material along the length of a tube in this manneris possible because of the interaction of the circumferential shearingaction on the longitudinal stress pattern in the direction of theextrusion the weight per inch of the tube is reduced whilst thecircumferential shearing action is taking place, due to the fact thatless longitudinal contraction of the polymer takes place whilst thecircumferential shearing action is applied.

The continuous or intermittent rotation of the core can be used todistribute the material more evenly in an article moulded from theextruded tube since Where a reduction in thickness is required the coremay be rotated at a greater speed thereby causing a reduction in theweight per unit length of the tube.

I claim:

1. A method of extruding a parison, whose weight per unit length,diameter and wall thicknes vary in the extrusion direction, forsubsequent inflation in a mould, comprising the steps of continuouslyextruding thermoplastic material through an annular die orifice having acore at a constant rate to form a tube, and applying to the wall of saidtube a series of brief tangential shearing forces insufficient to severthe wall by periodically rotating said core and die orifice relative toeach other, each of said shearing forces being applied for less than oneturn.

2. The method as claimed in claim 1 wherein each shearing force of saidseries is aplied by rotating said core about one quarter turn at a rateof approximately 20 r.p.m.

3. The method as claimed in claim 1 wherein each said shearing force ofsaid series comprises opposed circumferential frictional forces appliedto the inner and outer wall of said tube as it leaves the extrusionorifice, the successive shearing forces of said series being resultantsin the same direction of the said opposed forces.

References Cited by the Examiner UNITED STATES PATENTS 2,632,202 3/1953Haines 18-5 2,801,441 8/1957 Wadsworth 18-149 3,008,187 11/1961 Slade264119 3,020,588 2/1962 Ferguson et al. 18149 3,029,468 4/1962 Valyi18-5 3,059,277 10/1962 Pierce et a1 18149 ROBERT F. WHITE, PrimaryExaminer.

ALEXANDER H. BRODMERKEL, Examiner.

M. H. ROSEN, L. S. SQUIRES, Assistant Examiners.

1. A METHOD OF EXTRUDING A PARISON, WHOSE WEIGHT PER UNIT LENGTH,DIAMETER AND WALL THICKNESS VARY IN THE EXTRUSION DIRECTION, FORSUBSEQUENT INFLATION IN A MOULD, COMPRISING THE STEPS OF CONTINUOUSLYEXTRUDING THERMOPLASTIC MATERIAL THROUGH AN ANNULAR DIE ORIFICE HAVING ACORE AT A CONSTANT RATE TO FORM A TUBE, AND APPLYING TO